Diversity reception is widely used to stably receive terrestrial broadcasting on a mobile object such as an automobile or mobile telephone set.
Diversity reception method includes space diversity, which utilizes the difference in spatial arrangement between transmitting and receiving antennas; time diversity, which improves reception probability by transmitting the same signal multiple times; frequency diversity, which transmits the same signal in multiple frequency bands and utilizes the characteristic in which fading occurs differently depending on a frequency band, aiming at stable reception in any of the bands; and polarization diversity, which utilizes the difference in polarization characteristic among transmission signals.
Out of these methods, time diversity, frequency diversity, and polarization diversity require the transmission side to transmit the same information by multiple times or with multiple means. Accordingly, in order to improve reception characteristic for terrestrial broadcasting, space diversity is widely used that is implemented by changing the reception configuration at the receiving side for effectively utilizing the limited frequency resources.
When performing mobile reception of analog TV broadcasting on an automobile, for instance, space diversity reception is in practical use in which multiple antennas are set on an automobile to select an input signal with the highest level of received signal out of multiple received signals available.
Meanwhile, digitization of broadcasting is being promoted currently. In Japan and Europe, for example, Orthogonal Frequency Division Multiplexing (referred to as “OFDM” hereinafter) has been adopted as a terrestrial digital television broadcasting system.
Transmission data to be OFDM-modulated and transmitted undergoes information source coding based on MPEG2 information source coding.
An error correction process is performed to improve reception error tolerance when receiving signals. Further, changing carrier modulation method to that with higher error immunity improves error tolerance as well. Changing carrier modulation method from the modulation method called 64QAM to that called 16QAM, for example, improves noise immunity of a signal although the information rate of the signal to be transmitted decreases.
When applying space diversity reception for the signal OFDM-modulated mentioned above, multiple signals are received by multiple antennas and undergo A/D conversion, synchronous detection, FFT computing, and a demodulation process, for each received signal individually. Consequently, an OFDM signal composed of a large number of carriers is generated for each signal received by multiple antennas.
In space diversity reception, processing signals received by multiple antennas in the unit of OFDM carrier is most effective. More specifically, one optimum signal is selected from signals received by multiple antennas for each OFDM carrier, or signals received by multiple antennas are synthesized for each OFDM carrier.
When selecting one optimum signal, the power amount of an OFDM carrier, for example, is to be a selection criterion. In other words, the power amounts are compared for each OFDM carrier and the largest one is selected.
When synthesizing signals, a weighting amount is calculated for multiple signals obtained, for each OFDM carrier, and the signal are added according to a ratio calculated from the weighting amount. In this case, if the method called maximum ratio combining diversity reception is used, in which weighting is made according to the ratio of the power amount for each OFDM carrier, the noise amount for the signal can be minimized, and thus the reception characteristic is highly improved. Maximum ratio combining diversity reception is disclosed in the nonpatent literature “Linear Diversity Combining Techniques” Proc. IRE, 471075-1102, June 1959, by D. G. Brennan.
As mentioned above, if diversity reception method is used that demodulates multiple signals OFDM-modulated received by multiple antennas are to an OFDM carrier for each received signal, and selects or synthesizes the OFDM carriers obtained by the number of antennas, the power amount of an OFDM carrier is used as a selection criterion when selecting or a calculation criterion for a synthesizing ratio when synthesizing.
The power amount of an OFDM carrier is calculated from pilot carriers arranged in the OFDM signal at regular intervals frequencywise and timewise. The amplitude and phase of the pilot carriers are known, and thus the changes in amplitude and phase are regarded as having been given by the fluctuation in the transmission line to estimate a transmission line characteristic (the degree of deviation in amplitude and phase), and the power amount of all the OFDM carriers are calculated.
However, when a noise signal, particularly a frequency-selective disturbing signal, has been added to the pilot signal for an OFDM signal, the amplitude and phase of the pilot carrier fluctuate as well, causing an error in estimating a transmission line characteristic. Meanwhile, a signal transmitted with the data carrier of the OFDM signal is calculated by dividing the received data by the transmission line characteristic estimated from the pilot carrier. Accordingly, an error occurs in the signal of the data carrier as well.
Further, even when a noise signal with its characteristic different from that of an OFDM signal is added only to the data carrier signal for the OFDM signal, an error occurs in the data carrier signal. When using a signal OFDM-modulated and received by multiple antennas, with being selected or synthesized by means of space diversity reception, a signal with incorrect information results in being selected or synthesized if a signal received by at least one antenna is affected by the above-mentioned noise signal, potentially causing a deteriorated reception characteristic on the contrary.